Graduation Of Carsten Markus: Designing And Casting Of Impellers

Impeller under operating load.
Impeller under operating load.

Last week, Carsten Markus graduated on his assignment with our research department Damen Dredging Equipment. He investigated the alloys that are used to cast our impellers and the responses of the materials under operating conditions. We are always improving our dredge pumps and Carsten’s work has been a great contribution for our development.

For every dredging application, the material for the dredge pump parts can be carefully selected according the specific requirements on the sediment handled. Off course, one would like to have the hardest, most erosion resistant material available. Less erosion is less wear and a longer lifetime1.

Wear part material hardness in relation to wear index

However, there is a downside to choosing very hard material: it will be very brittle also2. Basically, the ultimate tensile stress coincides with the yield stress. There is no reserve for the load. When the stress surpasses the yield strength, it just snaps. Conversely, a tough material has a lot of reserve. After deformation beyond the yield stress, the load can still increase without a catastrophic failure. Usually, the stresses would not be that high, the thickness of the wear parts is dimensioned for erosion reserves and thick parts have low stresses. The high stresses can come from concentrations. Either when the wear reserves are eroded, or when a heavy load is concentrated on a very small area. Usually stones. Stones are a very common problem in a dredging project. So, next to the hardness of an alloy, also the toughness will be a very important characteristic. And toughness is related to the yield elongation after failure.

For the various materials you would like to know how the base load relates to the tensile stress. This would be an indication whether there is reserve in the elastic region to accommodate the impact of an occasional stone. This base load depends on many factors in the pumping process. Mainly the pressure generation in the mixture over the blades. Most known literature is about the force distribution in the volute of the pump, as this is directly related to the radial forces and consequently the bearing and shaft calculations. The CFD simulations in this graduation project revealed the skewed load on the volute and consequently the load variations on the impeller.

Transient CFD simulation of a dredge pump. The rotation appears wrong , but is correct (Wagon-wheel effect).

As emphasised before, it is very beneficial to operate your dredge system around the Best Efficiency Point (BEP). Not only the shaft and bearings suffer less, also the impact of the stresses and their variations in the impeller are less. If the operating point differs from the required capacity, the BEP can be moved by changing the dredge pump speed. As long as head requirements permit the adjustment.

Radial load as a function of pump characteristics.
Radial load as a function of pump characteristics.

As a result of Carsten’s research, we were able to improve our operating load models for the whole dredge pump and gained insight into the material responses to these. Thank you Carsten.

Due to the measures taken for the containment of the Corona virus, the graduation itself and the party to celebrate it, where done remotely over internet. Let’s enjoy the real beer later, after all this commotion is over. Stay home, stay safe.

Carsten’s graduation defence session under Covid-19 measures.
Carsten’s graduation defence session under Covid-19 measures.

References

  1. Do You Have Wear Parts For Spare?, Discover Dredging
  2. Brittleness, Wikipedia

See also

Don’t Play Games With Your Wear Part Planning

Board game for wear part manufacturing planning
Board game for wear part manufacturing planning

Last week we had another of our training courses for service engineers and field service engineers1. The interaction with people actually working with our products is quite refreshing and every time I understand their issues better. One of those issues is that they have to discuss with the client are spares for the wear parts. In a planned maintenance context, wear parts are a little odd. Sometimes, they are worn away or break down unexpectedly. And that is the moment customers call for spares. We do have a lot of spares on stock, but sometimes even we run out of stock or we advise to use a special execution of the concerned part for the specific operation of the client. And then we have to inform the service people and the client that there is a long lead time. Several times, they are filled with disbelief and under such circumstances it is very difficult to explain the reasons behind it. So, that is why I developed this little game to experience the waiting time for special wear parts.

Layout of the board for the wear part game
Layout of the board for the wear part game (Download pdf version here)

It is based on the old board game of ‘Snakes and Ladders’2. All it takes are the board, one dice and as much tokens as players. All start at the first position. The places are all phases in the manufacturing of the wear parts and each have their specific issues.

  1. Each pattern is used multiple times and wears down, itself. Also, some patterns have to be configured for the specific application, execution or material of the wear part.
  2. Moulding: the pattern is placed in a casting box and filled with sand.
  3. Sometimes there are more casting boxes needed and they have to be stacked carefully. Depending on the configuration, this step might be very short. Then you go directly from 2 to 4.
  4. Part of the casting system is already in the sand box with the pattern, but it has to be finished as the last part of the preparation.
  5. The material is melted in the furnace. This can take some time, depending on the size of the cast.
  6. The actual casting is done in minutes. Fifteen, at the most.
  7. But the cooling in the casting box takes weeks. Wait one turn.
  8. Sometimes the casting has not gone properly and the cast have to be done again. Back to square one.
  9. Satisfied with the cast, then it has to be touched up at the fettling station.
  10. A special heat treatment brings the final hardness and toughness to the product.
  11. The fitting surfaces of the wear parts have to be machined.
  12. Rotating parts have to be balanced. For non-rotating parts, this can be skipped.
  13. Then there is the bottle neck: quality control. If there is a deviation that can n ot be mitigated, you have to go back to square one.
  14. Depending on the location, transport can take weeks.
  15. Don’t start me about customs handling. Your anticipated spares are in bonded storage and customs is missing a document, wait some weeks or skip a turn.
  16. Finally, you’ve made it! Installation on the dredge.

Message of the game: keep your warehouse well stocked with wear parts3,4, or your dredge will be idle for months, before you can work again. Have fun!

Spare parts on stock
Spare parts on stock

References

  1. A well-trained team makes all the difference, Damen
  2. Snakes and Ladders, Wikipedia
  3. Do You Have Wear Parts For Spare?, Discover Dredging
  4. Options for Repairing Parts That Ought to be Replaced, Discover Dredging

See also

Graduation Suman Sapkota: Where wear parts were worn down

Suman Sapkota signing his Master of Science degree

Yesterday, Suman Sapkota graduated at the University of Twente on: ‘Technical and Sustainability Analysis of Sediment Erosion of Impeller Blades of Dredge Pumps’. We are very proud he worked for us on this topic and graduated with a grade 8 for it. As such, he stands in a long line of Master students1, who graduate with an 8 or even higher2. We value good students and we like to work with them only if the work they deliver is useful for us. This requires an intensive supervision and the results are correspondingly. This does not guarantee a good result, but you can always try3.

Suman graduated on a topic covered by the chair of ‘Sustainable Energy Technology’4. The objective was to study the economic and environmental impact of worn wear parts. When wear parts have to be rejected too early, they increase their environmental footprint. On the other end, if an impeller is severely worn, the efficiency decreases and the environmental footprint increases also. Our question was whether it was possible to improve the design of the dredge pump for a longer lifetime by checking the wear rate of the improvement in a simulation. To understand the problem and answer the question Suman had to start investigating the wear process itself.

Introduction sediment erosion in dredging applications

From literature he evaluated different erosion models. Basically, what is the effect when a defined particle impacts the concerned surface under certain conditions. And the conditions will be depending on the flow pattern and the influence of the fluid on the particle trajectories. The second research part was the investigation of this flow pattern and simulated with a CFD application. The flow pattern and the particle properties were combined. This way, the movement of the particles can be traced from inlet to outlet.

Particle tracking in CFD simulation of an impeller

Between inlet and outlet, the particles bounce against several areas of the blade. The bouncing can be counted and mapped. Together with the information of the impingement conditions and the erosion model, an estimation of the erosion rate can be made for different flow conditions and soil properties. The mapping can be transformed into an erosion density map. These maps can be checked for pump capacity and particle sizes.

Sediment erosion rate density map

With these maps, we can finally evaluate in advance where impellers will wear down and if we can improve their lifetime by modifying the design. Thus we can reduce their environmental footprint and contribute to a more sustainable business.

Suman, thank you very much for working for us. You were a pleasure to work with. We wish you good luck on your career and a happy life.

Congratulations to Suman Sapkota. Well done!

References

  1. Graduation reports, Discover Dredging
  2. Ben Sloof Nominated For KIvI Best Offshore Graduate Student
  3. Internship Vacancies, Damen
  4. Kipaji scholarship for Nepalese student, University of Twente

See also